1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device in which reflective portions and transmissive portions are provided in a liquid crystal cell.
2. Description of the Related Art
In the field of a display device, an active matrix display device capable of obtaining high display quality has come into widespread use. In this display device, a switching element is provided in each pixel electrode to perform reliable switching therein, which makes it possible to achieve a display device having a large size and high accuracy.
In recent years, in the display device, a reduction in power consumption has been demanded, and an increase in the area of a pixel region has been also required to improve the brightness of display. Therefore, in order to fulfill the requirements, generally, a thick insulating film is formed on the entire surface of an active matrix substrate, and reflective pixel electrodes are formed on the insulating film. The structure in which the pixel electrodes are formed on the insulating film makes it possible to prevent a short circuit between scanning lines or signal lines arranged below the insulating film and the pixel electrodes formed on the insulating film, and thus to form the pixel electrodes on these wiring lines with a large area so as to overlap them. In this way, it is possible to allow almost the entire region including a region where switching elements, such as thin film transistors (hereinafter, referred to as TFTs), the scanning lines, and the signal lines are formed to serve as pixel regions contributing to display, and thus to raise an aperture ratio, which results in bright display.
However, the liquid crystal display device using only the reflective pixel electrodes cannot be used in a dark place. Therefore, there has come into widespread use a transflective liquid crystal display device having a backlight additionally provided therein, which enables a reflective liquid crystal display device to partially perform transmissive display (see JP-A-2000-171794 and Japanese Patent No. 3235102).
FIG. 13 shows an example of a transflective liquid crystal display device according to the related art (see JP-A-2000-171794). In the liquid crystal display device shown in FIG. 13, a plurality of TFTs 111 is formed in a display region on a transparent substrate 110, and an insulating film 112 is formed on the TFTs 111. In addition, reflective pixel electrodes 113 composed of an aluminum electrode film is formed on the insulating film 112 at pixel forming positions, and an uneven portion of each pixel electrode 113 serves as a reflective portion. Further, concave portions 116 are formed in the insulating film 112 positioned below these pixel electrodes 113, and a transparent gate insulating film 118 and a transparent drain electrode 119 are sequentially formed on the bottom of each concave portion 116. In this structure, a portion of the concave portion 116 where the drain electrode 119 is formed serves as a transmissive portion a. In addition, an alignment film 122 is formed so as to cover the insulating film 112, the pixel electrodes 113, and the concave portions 116.
Furthermore, a substrate 120 is provided opposite to the substrate 110 with a liquid crystal layer 109 interposed therebetween. A transparent counter electrode (a common electrode) 121 and an alignment film 123 are sequentially formed on a surface of the substrate 120 facing the liquid crystal layer 109.
However, a backlight provided on the rear surface of the substrate 110 is not shown in FIG. 13.
In the transflective liquid crystal display device shown in FIG. 13, an electric field is applied from the reflective pixel electrodes 113 and the source electrodes 119 to the liquid crystal layer 109 provided between the substrates to control the alignment of liquid crystal, thereby performing display. More specifically, a voltage is applied to the reflective pixel electrode 113 through the TFT 111 to control the alignment of the liquid crystal, so that the transmittance of the liquid crystal is controlled. The uneven portion of the pixel electrode 113 serves as a reflective display region which reflects light incident from the substrate 120 to perform reflective display. Also, a voltage is applied to the transparent source electrode 119 in addition to the pixel electrode 113 through the TFT 111, and a part of the concave portion 116 where the source electrode 119 is formed serves as a transmissive display region that transmits light emitted from the backlight to perform transmissive display. That is, a transflective liquid crystal display device capable of performing both the reflective display mode by the reflective pixel electrodes 113 and the transmissive display mode using the backlight and the transparent electrodes 119 is achieved by the structure shown in FIG. 13.
Further, in the transflective liquid crystal display device, light incident on the liquid crystal display device passes through the liquid crystal layer two times to reach an observer in the reflective display mode. On the other hand, the light passes through the liquid crystal layer only one time in the transmissive display mode. Therefore, unnecessary colors may be mixed with a display color, or different display colors may appear according to the display mode. In order to solve the problems, in the structure shown in FIG. 13, the liquid crystal layer is formed with a small thickness d1 in the reflective portion, but is formed with a large thickness d2 in the transmissive portion a due to the depth of the concave portion 116 formed in the insulating film 112 (a dual gap structure in which a cell gap of the reflective display region differs from that of the transmissive display region). In addition, the thickness (optical path) of the liquid crystal layer when light passes through the liquid crystal layer two times in the transmissive display mode is set to be equal to the thickness (optical path) of the liquid crystal layer when light passes through the liquid crystal layer one time, which makes it possible to prevent the mixture of display colors or a variation in a display color due to a difference between optical paths of two display modes.
However, in the transflective liquid crystal display device shown in FIG. 13, as described above, the thickness d2 of the liquid crystal layer in the transmissive portion a (the liquid crystal layer in the transmissive display region) is two times the thickness d1 of the liquid crystal layer in the reflective portion. Therefore, a difference between the thicknesses of the liquid crystal layer in both the display modes becomes large, which causes a large difference between threshold values or saturated voltages of the liquid crystal layer in the transmissive display region and the reflective display region when a driving voltage is applied, resulting in a large difference in contrast between the transmissive display mode and the reflective display mode in the same pixel.